The race for presidential delegates has begun. Track them here.

The magic number to win the Democratic Party’s nomination for president is 1,991 delegates. It could take months to officially get there.

Presidential candidates are all competing for a majority of 3,979 pledged delegates. Separately, there are also 771 automatic delegates, otherwise known as “superdelegates.” But after a contentious 2016 primary, the Democratic National Committee changed its rules around superdelegates so they may now vote on the first ballot to select a nominee at the Democratic National Convention only if a campaign has secured a supermajority of pledged delegates. (Otherwise, they weigh in in the event of a second round of voting at the convention.)

Candidates are competing for these delegates against a backdrop of complex rules: Delegates are awarded proportionally, and most states and districts require a minimum threshold of 15 percent of the primary or caucus vote to earn pledged delegates. This makes the contest a bit fairer, but also means it’s much harder to secure that magic number in a contested primary until much later in the year.

The Iowa caucuses officially kicked off the delegate race. But while the early states are all about gaining momentum, the race is ultimately about who can bag the most delegates and get to that magic number.

This could all be over very quickly if one candidate racks up an insurmountable lead and emerges from a crowded pack, or it could drag on into the spring and summer, ahead of the Democratic National Convention in July.

Vox has a delegate tracker, in partnership with our friends at Decision Desk and the Virginia Institute of Politics.